Budget carrier Norwegian Air Shuttle’s plans to expand low-cost flights to U.S. have faced significant delays

The European Union took the unprecedented step Thursday of accusing Washington of breaching the so-called Open Skies pact aimed at liberalizing trans-Atlantic air travel—opening a high-profile trade rift just before the Trump administration takes office.

The challenge centers on a relatively obscure application by European budget carrier Norwegian Air Shuttle ASA, which is seeking to expand low-cost flights to the U.S. and has faced...